A fuel cell is a cell in which hydrogen, methanol or the like is electrochemically oxidized and thereby the chemical energy of fuel is directly converted into an electric energy and taken out, and this is attracting attention as a clean electric energy supply source. In particular, a solid polymer electrolyte fuel cell works at a low temperature as compared with others and is expected to be an automobile alternative power source, a domestic cogeneration system, a portable generator or the like.
The solid polymer electrolyte fuel cell comprises at least a membrane electrode assembly in which a gas diffusion electrode obtained by stacking an electrode catalyst layer and a gas diffusion layer is joined on each of both surfaces of a proton exchange membrane. The proton exchange membrane as used herein means a material having a strongly acidic group such as sulfonic acid group and carboxylic acid group in the polymer chain and having a property of selectively passing a proton. The proton exchange membrane which is suitably used is a perfluoro-based proton exchange membrane as represented by Nafion (registered trademark, produced by du Pont) having high chemical stability.
During the operation of a fuel cell, a fuel (e.g., hydrogen) is supplied to the gas diffusion electrode on the anode side, an oxidizing agent (e.g., oxygen, air) is supplied to the gas diffusion electrode on the cathode side, and both electrodes are connected through an external circuit, thereby actuating the fuel cell. More specifically, when the fuel is hydrogen, the hydrogen is oxidized on an anode catalyst to produce a proton, and this proton passes through a proton conductive polymer in the anode catalyst layer, then moves in the proton exchange membrane and passes through a proton conductive polymer in the cathode catalyst layer to reach on the cathode catalyst. On the other hand, an electron produced simultaneously with the proton by the oxidation of hydrogen passes through the external circuit to reach the gas diffusion electrode on the cathode side and reacts with the proton and oxygen in the oxidizing agent to produce water, and an electric energy can be taken out at this time.
In this case, the proton exchange membrane must act also as a gas barrier and if the gas permeability of the proton exchange membrane is high, the hydrogen on the anode side leaks toward the cathode side and the oxygen on the cathode side leaks toward the anode side, that is, a cross leakage is generated, as a result, a so-called chemical short state is produced and a good voltage cannot be taken out.
The solid polymer electrolyte fuel cell is usually operated at around 80° C. in order to bring out high output properties, but in usage for automobiles, assuming travel of an automobile in the summer season, the fuel cell is required to be operable even under high-temperature low-humidification conditions (an operation temperature in the vicinity of 100° C. with 50° C. humidification (corresponding to a humidity of 12 RH %)). However, when a fuel cell using a conventional perfluoro-based proton exchange membrane is operated for a long time under high-temperature low-humidification conditions, this causes a problem in that pinholes are generated in the proton exchange membrane and cross-leakage is brought about, and sufficiently high durability is not obtained.
With respect to the method for enhancing the durability of the perfluoro-based proton exchange membrane, studies have been reported to enhance the durability by the reinforcement using a fibrillated polytetrafluoroethylene (PTFE) (see, Japanese Unexamined Patent Publication (Kokai) No. 53-149881 and Japanese Examined Patent Publication (Kokoku) No. 63-61337), the reinforcement using a stretched PTFE porous film (see, Kokai No. 8-162132), the reinforcement of adding inorganic particles (see, Kokai Nos. 6-111827 and 9-219206 and U.S. Pat. No. 5,523,181), or the reinforcement using a porous body comprising an aromatic ring-containing resin (see, Kokai Nos. 2001-514431 and 2003-297393). However, in these methods, durability sufficiently high to solve the above-described problems cannot be achieved.